Addition of sugar to concrete results in:
Correct Answer: B. Increase in setting time by about 4 hours
📚 Detailed Explanation: Sugar Retards Setting by ≈4 Hours
Why B (Increase by about 4 hours) is correct: Sugar (saccharose/sucrose) is a very effective retarder for cement hydration. At a dosage of approximately 0.2% by mass of cement, it delays the initial setting time by about 4 hours. This is because sugar molecules adsorb strongly onto C3S and C3A surfaces in the cement particles, forming a thin coating that physically blocks water from reaching the unhydrated surface.
Sugar as a Retarder
| Dosage (% by wt. cement) | Effect on Setting Time | Practical Outcome |
|---|---|---|
| 0.05% | Delay of ≈1–2 hours | Mild retardation; slight extension |
| 0.1% | Delay of ≈2–3 hours | Moderate retardation |
| 0.2% | Delay of ≈4 hours | Significant retardation; the standard reference dosage |
| 0.3%+ | May cause permanent inhibition (never sets) | Excess sugar can permanently prevent setting — dangerous |
Mechanism of Sugar Retardation
| Step | What Happens |
|---|---|
| 1. Sugar dissolves | Sugar dissolves in mix water; produces sucrose molecules in solution |
| 2. Adsorption on cement | Sucrose molecules adsorb strongly onto C3S and C3A particle surfaces |
| 3. Barrier formation | Adsorbed layer prevents water from accessing the reactive cement surfaces |
| 4. Delayed hydration | C-S-H gel formation slowed; setting time extended by ≈4 hours |
| 5. Sugar gradually deactivated | As Ca2+ ions build up, they displace sucrose; hydration resumes normally after delay |
Practical Uses of Sugar as a Retarder
| Situation | Why Sugar Retarder Is Used |
|---|---|
| Emergency site situations | Sugar is cheap and available; can buy time if ready-mix truck is delayed |
| Hot weather concreting | Combats the accelerating effect of high temperature |
| Exposed aggregate finish | Applied to the surface; retards surface paste; wash off later to expose aggregate texture |
- Sugar at 0.2% by weight of cement increases setting time by approximately 4 hours.
- Excess sugar (>0.3%) can permanently prevent setting — a common cause of concrete failures when laborers add too much.
- Sugar is an economical emergency retarder; it acts by adsorption onto cement particle surfaces.